1. Field of the Invention
The present invention relates to distortion-compensation amplification apparatuses and distortion compensation methods, and particularly to a distortion-compensation amplification apparatus for compensating for distortion of a transmission signal and for amplifying and outputting the transmission signal and a distortion compensation method therefor.
2. Description of the Related Art
In wireless communication, high-efficiency transmission by digitalization has increased in recent years. What is important in wireless communication using a multilevel phase shift keying system is a technology for reducing adjacent channel leakage power by making the amplification characteristics of the transmitting power amplifier linear on the transmitter side such as a base station to suppress non-linear distortion. If an amplifier having poor power linear characteristics is used to improve the power efficiency, a technology for compensating for the resulting non-linear distortion is necessary (refer to Japanese Unexamined Patent Application Publication No. 2006-270246, for example).
A general distortion-compensation amplification apparatus performs peak suppression to suppress a distortion component (refer to Japanese Unexamined Patent Application Publication No. 2004-64711, for example). The peak suppression blocks the input of a signal not smaller than the saturation power, thereby compensating for distortion.
FIG. 10 illustrates the amplification characteristics of a distortion-compensation amplification apparatus. In the figure, the horizontal axis represents the input level of the signal input to the distortion-compensation amplification apparatus, and the vertical axis represents the output level of the signal output from the distortion-compensation amplification apparatus.
A waveform W101 in the figure represents ideal amplification characteristics of the distortion-compensation amplification apparatus. The relationship between the input signal and the output signal of the distortion-compensation amplification apparatus is desired to be linear as represented by the waveform W101, so that an amplified signal can be output without distortion.
A waveform W102 in the figure represents amplification characteristics of the distortion-compensation amplification apparatus when distortion compensation is not performed. As the waveform W102 indicates, the distortion-compensation amplification apparatus maintains linearity for the input signal up to a certain level and loses the linearity beyond the certain level. If a signal of a higher level is input, the distortion-compensation amplification apparatus is saturated in gain and outputs a signal of a uniform level (as represented by the flat part in the waveform W102).
A waveform W103 in the figure represents the relationship between the input level and output level of the distortion-compensation amplification apparatus when distortion compensation is performed. As described earlier, when distortion compensation is not performed, the amplification characteristics of the distortion-compensation amplification apparatus becomes non-linear for the input signal exceeding a certain level. The distortion-compensation amplification apparatus obtains linearity, as represented by the waveform W103, by raising the level of the input signal in the non-linear part of the amplification characteristics (the non-linear part of the waveform W102). To be more specific, the distortion-compensation amplification apparatus obtains a linear amplification characteristic such as that represented by the waveform W101, by multiplying the input signal by such a distortion compensation coefficient that the output signal becomes linear.
However, if a signal not smaller than the saturation point of the distortion-compensation amplification apparatus is input, an increased distortion compensation coefficient cannot increase the output signal beyond a certain level. This would cause the distortion compensation coefficient to be updated toward an infinite value and to diverge in the gain saturation part. Accordingly, the level of the input signal to the distortion-compensation amplification apparatus should not exceed a dotted line D101 in the figure, at most. This means that the distortion-compensation amplification apparatus should perform peak suppression so that the input signal will not exceed the level indicated by the dotted line D101 in the figure.
A dotted line D102 in FIG. 10 represents the operating point of the distortion-compensation amplification apparatus. The ratio of the operating point to the peak suppression point is referred to as a peak average rate (PAR).
FIG. 11 illustrates peak suppression. A dotted line D111 in the figure represents the peak suppression point. The value of the peak suppression point represented by the dotted line D111 is held to the value of the dotted line D101 in FIG. 10, for instance, so that the distortion compensation coefficient will not diverge.
If an input signal exceeds the peak suppression point, as represented by X1(t) in the figure, the distortion-compensation amplification apparatus performs peak suppression, as represented by X2(t) in the same figure. Through this processing, the distortion-compensation amplification apparatus prevents the distortion compensation coefficient from diverging.
A dotted line D112 in the figure represents the average power of the input signal X2(t). The distortion-compensation amplification apparatus is designed to set the average power of the input signal X2(t) as its operating point.
FIG. 12 shows the relationship between the error vector magnitude (indicating modulation precision) and the peak suppression point. In the figure, the horizontal axis represents the peak average rate (PAR), and the vertical axis represents the error vector magnitude (EVM).
As shown in the figure, an increase in peak average rate improves the EVM (error vector magnitude). A standard range of EVM is specified, and the peak suppression point must be set corresponding to γ or more in the shown example.
FIG. 13 illustrates adjacent channel leakage power. In the figure, the horizontal axis represents frequency, and the vertical axis represents power. In this figure, P101 to 104 represent the power of transmission carriers, and P111 to P114 represent adjacent channel leakage power.
A standard adjacent channel leakage power is specified. The leakage power must be −45 dB or lower at ±5 MHz from the transmission carrier frequency band and must be −50 dB or lower at ±10 MHz from the transmission carrier frequency band, for example.
However, the peak suppression, which changes the waveform of the transmission signal as shown in FIG. 11, may cause degradation in error vector magnitude. For instance, if a low peak suppression point is specified (if the dotted line D111 in FIG. 11 is lowered), the error vector magnitude would deteriorate to such a level that the EVM standard cannot be satisfied.
If a high peak suppression point is specified (the dotted line D101 is specified rightward in FIG. 10) to improve the error vector magnitude, the amount of distortion of the transmission signal would increase beyond the specifications of adjacent channel leakage power.
Accordingly, in a conventional distortion-compensation amplification apparatus, a margin of several decibels is provided between a gain saturation point (where the waveform W102 in FIG. 10 becomes flat) and a peak suppression point, in consideration of the temperature characteristics and the like. In other words, the flat part of the waveform W102 is raised to improve the linearity. This approach, however, increases power consumption.